Fitting Bigger Tank 1999 GMC Sierra

Greetings all. My uncle has a 1999 GMC Sierra 1-ton dually crew cab short bed truck with the 7.4L big-block. Oddly, this behemoth only came with something like a 23 gallon tank! From what I've been told, it can only get 150 miles or so per tank because the tank's so small, especially for that big motor. Will a factory tank that's bigger fit on the truck as a bolt-on? Something perhaps in the range of closer to 40 gallons? Oddly enough, the original windows sticker for this truck lists the fuel tank as something around 37 gallons, though the RPO codes and visual inspection confirm it to be the substantially smaller 23 gallon tank. Thanks for the help!

Crew cab dually short bed doesn't seem quite right. I didn't think chevy ever made anything like that from the factory!

First thing to find out is if it actually is a pickup or some sort of conversion built on the cab and chassis platform. They take totally different style fuel tanks. The easiest way to tell since you know the tank only holds 20-some gallons is to measure the length of the tank. If the tank is about 48" long, its made on the pickup chassis. If the tank is 64" long, its the cab and chassis. Both tanks have similar capacities (25 gallons for the 48" long tank, 22 for the 64" long) but are completely different in how they mount and their sending units.

If the truck is made on the pickup platform, there is a larger 34 gallon tank available that will fit, you'll just have to move the main brackets holding the tank to the frame. The sending unit and the fuel pump are the same. If the truck is a cab and chassis, sorry to say there's no easy way to do it.

The parts guy at GMC argued over and over with my uncle that a crew cab short bed dually doesn't exist, but it's definitely built from the factory that way, it wasn't a conversion. I'll have to take some pics of it, and measure the tank. Luckily, or unluckily, pick your poision, the tank is out of the truck so I can easily measure it. Thanks for the info!